Allow to set apply-only per cluster via config item

[linki]

Sometimes you want to have CLM update manifests as fast as possible and skip and defer any node rolls. CLM has a global flag for that --apply-only but it's rarely used since it requires to redeploy CLM and it's global.

This adds a config item, similar to update_strategy, that allows to configure the apply-only behaviour at runtime on a per-cluster basis.

When a cluster is stuck in a long node roll, the way it's supposed to be used is like that:

// set apply only to true
$ zregistry clusters config-items set -k update_apply_only -v true --cluster-alias <cluster>
// briefly block cluster update to retrigger CLM
$ echo "retrigger CLM" | zkubectl --context <cluster> cluster-update block
// wait a couple of seconds, then unblock the cluster again
$ echo y | zkubectl --context <cluster> cluster-update unblock
// once all the manifests have been applied, set apply only back to the original to start rolling nodes again
$ zregistry clusters config-items delete -k update_apply_only --cluster-alias <cluster>

Note: don't merge, untested

[AlexanderYastrebov]

// briefly block cluster update to retrigger CLM

Why is this needed? I think CLM will be triggered by the update_apply_only config item change, won't it?

[linki]

@AlexanderYastrebov Yes, if there's no update in progress. The example specifically handles the case where a long-running cluster update with node rotations is already underway. In this case changing the config item will not retrigger CLM until the node rotations are done which can take a long time. Restarting CLM or blocking/unblocking does the trick then.